1. Field of the Invention
The present invention relates to an apparatus for measuring a depth and a gradient of a trench in a semiconductor device, and more particularly, to an apparatus for measuring a depth and a gradient of a trench in a semiconductor device which is capable of accurately measuring a depth and a gradient of a trench actually on a wafer without damaging it, and to its method.
2. Description of the Background Art
Generally, as an isolating method of a highly integrated semiconductor device, a structure is taken that a trench is formed, in which an oxide film is filled. Therefore, forming the trench and measuring its depth and gradient is a requisite step to obtain a reliability for the process of the semiconductor device.
An apparatus for measuring characteristics of a semiconductor pattern uses an optical path difference between a light reflected from the surface of a test sample and a light having passed a medium. This is effective in measuring a thickness of a stacked film, which, however, is not suitably applied in case that the substrate is partially etched to form a step in an arbitrary depth, such as the trench.
For this reason, in order to measure the depth of the trench, the wafer is cut to observe its section, a method using a measuring instrument of xcex1-step is employed, or a method using an equipment xe2x80x98AFMxe2x80x99 for measuring a roughness of a surface is employed.
An apparatus and a method for measuring a depth and a gradient of a trench in a semiconductor device in accordance with a conventional art will now be described with reference to the accompanying drawings.
FIG. 1 is a flow chart of a process of a method for measuring a depth and a gradient of a trench in a semiconductor device in accordance with a conventional art, which employes the method that the wafer test sample where a trench is formed is cut and its section is observed, including the steps of: preparing a section-observing test sample wafer for observing a section besides an actual wafer for forming a semiconductor device; forming a trench on the section-observing test sample wafer by a photolithographic process; and cutting the section-observing test sample wafer in a manner that the position where the trench is formed is shown in the cross section, and observing a depth and a gradient of the trench.
In this respect, as to the depth-measuring test sample wafer, the trench is to be formed the same as the actual product, and after it is cut, its section is observed by using an electron microscope SEM.
However, such an testing method is a kind of destructive test that needs to cut the wafer, which is not able to measure a depth and the gradient of the trench formed on the actual wafer where the semiconductor device is formed, degrading a reliability and causing an increase of expense due to the additional fabrication of he test sample wafer.
FIG. 2 is a schematic view of a method for measuring a depth of a trench by using an xcex1-step equipment.
As shown in this drawing, a probe needle 1 is moved on a wafer 2 where a trench is formed, so as to measure a depth of the trench by a piezoelectric transducer by using a pressure difference between the upper portion of the wafer 2 where the trench is not formed and the lower portion of the trench.
In case of using the probe needle 1, since a pressure is high at the upper surface of the wafer 2 where a trench is not formed, while it is relatively low at the lower surface of the trench where the step is low, the depth of the trench can be calculated by using the pressure difference. But, since the wafer is damaged while the probe needle is being moved, this method can not be adopted to the actual wafer to fabricate a semiconductor device, and thus, a specific test sample needs to be separately made like in the above method for observing the section.
FIG. 3 is a schematic view showing a method for measuring a depth of a trench by using an AFM equipment in accordance with a conventional art.
As shown in this drawing, the AFM equipment measures a roughness of the wafer 2 by using the probe needle 1 at a position separated by 100 xc3x85 at maximum from the upper surface of the wafer.
Unlike the above described two conventional embodiments, this measuring method uses a Van der Waals"" force that works on the mutually separated probe needle 1 and the water 2 so as to numerically express a variation in the force working on a separated distance, thereby measuring the roughness of a specific thin film. This is a non-destructive testing which advantageously does not do a direct damage to the wafer.
However, the ATM equipment is applied to measure a roughness with deviation of tens of nm, which is not suitable to measure a depth of a trench formed in deviation of hundreds of nm in the process for fabricating semiconductor device.
Noticing from a graph of FIG. 4 showing a variation of Van der Waals"" force depending on a distance, as the distance between the probe needle 1 and the wafer 2 becomes close, a repulsive force works on therebetween, while the distance between the two becomes more distant, an attractive force works on therebetween. In this respect, in case of more than a predetermined distance, the attractive force is maintained at the same value in spite of the variation in the distance. Then, measurement of a depth would be impossible for a structure having a depth longer than a predetermined depth.
In addition, except the method for observing the section by cutting the wafer, other methods are not suitably applied to characteristic test for a trench in terms of accuracy because the gradient of the trench can be hardly measured.
Thus, currently in order to test the characteristics of the trench formed on the wafer, a method is yet largely taken in that a trench is formed on the test sample, which is then cut to observe a section thereof.
Accordingly, as to the apparatus and method for measuring the depth and the gradient of the trench in the semiconductor device, since the depth and the gradient of the trench formed on the actual wafer for forming the semiconductor device can not be measured, a test sample is to be additionally fabricated, causing an increase in expense and degrading a reliability.
Especially, methods for attempting to measure the gradient of the trench is very limited, causing a problem in that characteristics of the trench can not be accurately measured, resulting in degradation of the semiconductor device.
Therefore, an object of the present invention is to provide an apparatus for measuring a depth and a gradient of a trench in a semiconductor device which is capable of accurately measuring a depth and a gradient of a trench on an actual wafer without damaging it, and to its method.
To achieve these and other advantages and in accordance with the purposed of the present invention, as embodied and broadly described herein, there is provided an apparatus for measuring a depth and a gradient of a trench in a semiconductor device including: a light emitting element for making a light incident on a wafer while varying an incident angle, the wafer having a trench structure carried by a test sample carrying unit; a detect unit for detecting a light reflected from the wafer at a position symmetrical to the light emitting element on the basis of a virtual vertical line perpendicular to the wafer; a photoelectric conversion unit for converting an intensity of a light detected by the detect unit to an electric signal; an operating unit for receiving an output signal from the photoelectric conversion unit, catching a time point where the size of the received signal is increased or maintained, computing a depth of the trench by using a critical angle, that is, an incident angle where the incident light is reflected from the central portion of a lower surface of the trench, and judging the time point when the increasing output signal of the photoelectric conversion unit is maintained when the incident angle is below a critical angle as a sloping angle of the trench; and a display unit for displaying an operating result of the operating unit.
In order to obtain the above object, there is also provide a method for measuring a depth and a gradient of a trench in a semiconductor device including: a test-sample preparing step for preparing a test sample trench on a wafer, where a semiconductor device is to be formed; critical angle and sloping angle detecting step for making a light, of which incident angle is sequentially varied, incident on the wafer where the trench was formed, detecting a light reflected from the wafer, and detecting both a critical angle in which the incident light is first reflected from the bottom of the trench and an incident angle at the time point when the intensity of the incident light is maximized; and a trench depth computing step for computing a depth of the trench by using the critical angle and the width of the trench.